Baie de Somme

The Somme estuary covers around seventy square kilometres of sand, mudflat and salt marsh, emptying and filling on a tidal range large enough that the water withdraws several kilometres. France's largest colony of harbour seals hauls out on the banks, along with grey seals, and the bay is a major stopover on the Atlantic flyway with the Marquenterre reserve managing part of it. Crossing the bay on foot is a local tradition, done only with a guide because the channels shift and the tide returns fast. A metre-gauge steam railway on lines opened in 1887, run as a heritage line since the early 1970s, circles the bay between the towns.

What you actually see

• The seal colony - France's largest harbour seal population on the banks.
• The guided crossing - walking the bay at low tide with a guide.
• The Marquenterre reserve - hides and trails through dune and marsh.
• The steam railway - a preserved metre-gauge line around the bay.

When to go

April to October; seal pupping runs through summer and migration peaks in spring and autumn. Crossings depend entirely on tides and must be booked with a guide.

Honest expectations

Crossing without a guide is genuinely dangerous; the tide returns faster than people expect and there have been deaths. Seals must be watched from a distance and approaching them is prohibited. Weather on the bay is exposed.

Nearby

Saint-Valery lies on the southern shore; Le Crotoy is north.